Detection of underground objects has long been a very active research subject because of its important applications in mine detection, geological exploration, forensic investigation, treasure hunting, etc. Recent emphasis on environmental clean-up and remediation, has increased interest in this area. In converting abandoned military bases to civil use and in cleaning old battle fields, objects left in the ground, such as shells, unexploded ordnance (UXO), etc., need to be detected and removed.
Most of the techniques and systems currently available for the detection of underground objects are of the type which search the designated area and indicate the possible existence of some underground objects. They lack the capability to precisely locate and characterize the objects.
The difficulty in characterizing underground objects with electromagnetic (EM) methods stems from the ground medium surrounding the objects. Unlike air which is transparent and almost lossless for EM wave propagation, the ground is conductive and generally inhomogeneous causing an increase in signal dissipation and localization errors. Since dissipation increases with the EM frequency, it limits the use of the EM energy to low frequency bands when depth penetration beyond several meters is desired.
In air, the electromagnetic field satisfies the Helmholtz wave equation and operates in a propagation mode at distances between the source and the receiver much greater than a free space wavelength. The amplitude of the fields either remains constant or changes relatively slowly, but the phase changes rapidly with distance. From the phase delay of an electromagnetic field reflected from a target one can tell the range between the target and the observer. The directivity, or focused direction, along which the antenna transmits and receives the electromagnetic field gives the direction of the target. The range combined with the direction indicates the location of the target.
In contrast, in the ground, at measurement ranges much less than a wavelength, the electromagnetic field basically satisfies a diffusion type equation in the low frequency limit, rather than the usual wave equation as in the air. For distances much less than a wavelength, the phase delay, related to the spatial variation of the waves (i.e., e.sup.i2.pi.x/.lambda.), is too small to be used to measure the object's range and shape or to resolve multiple objects. In addition, the small size of the transmitter and receiver aperture relative to a wavelength does not provide a capability for measuring direction to the buried object.
Because of the potential danger involved in the remediation of abandoned military bases and old battlefields, not just the detection of the underground objects is required, but a more precise characterization which includes location, orientation, size, shape, and material composition is desirable. New technological systems which can satisfy these requirements are in urgent demand.